The present invention relates to bass reflex speaker systems with ducted ports. Such speaker systems are well known in the art of loudspeaker design, and have been sold and used in the USA since 1938. A bass reflex system provides improved efficiency and lower frequency limit than a speaker with a closed cabinet. This is because it acts as a Helmholtz resonator, which supplies low frequency sound waves from the rear of the driver to the outside of the cabinet in phase with the direct sound waves from the front of the driver. The desired resonance frequency is determined by the air mass in the ducted port and the compliance of the air volume in the cabinet.
The duct alone, however, also can act as a resonator for sound waves with half-wave length equal to the length of the duct or a fraction thereof. This is an undesirable effect, because frequencies corresponding to such resonances will pass from the inside of the cabinet to the outside, and will color the midrange sound of the speaker. The audible effect of such undesirable resonances can be reduced or eliminated by forcing the sound across different parts of the cross section of the duct to travel different distances, or by adding a low-pass filter after the duct.
The first type of solution can be approximated by using a duct that is sharply bent. Another example of this type of solution is described in U.S. Pat. No. 4,933,982, which uses a straight duct containing coaxial inserts to force the sound waves to travel different distances between the input and exit openings. Both types of duct are, however, expensive to make, and the latter is quite bulky.
An example of the second type of solution is described in U.S. Pat. No. 4,953,655, where a bass reflex duct terminates in a separate chamber with a port to the outside of the cabinet. The separate chamber with its port acts as a low pass filter, which removes resonances in the duct before the sound from the duct is allowed to reach the outside of the speaker cabinet. The size, the complexity, and the cost of the speaker cabinet, however, are increased by the extra chamber.